Fluorescent lamp gas filling



PELA T/VE Lz/M/Nous EFF/C/ENCY Q Dec. 5, 1961 K. SCHMIDT FLUORESCENT LAMP GAS FILLING' Filed May 11, 1959 ENVELOPE WALL TEMPEEA TUBE DEGREES CENT/GRADE x 2.0mm

PELA T/VE Lz/M/Naus fFF/C/ENCY Q ENVELOPE WALL 7EMPE/2AT1/2E DEGREES CENT/GEADE lnven tor KUT'CL' Schmid L',

United States Patent 3,012,165 FLUORESCENT LAMP GAS FILLING Kurt Schmidt, Cleveland, Ohio, assignor to General Electric Company, a corporation of New York Filed May 11, 1959, Ser. No. 812,236

' 6 Claims. (Cl. 313-185) This invention relates to low pressure electric discharge lamps such as fluorescent lamps using a mixture of mercury and inert gas as the ionizable medium.

In the fluorescent lamp, the electric discharge through a mixture of mercury vapor at a few microns pressure and an inert gas or mixture of inert gases at a few millimeters pressure produces ultraviolet radiation principally at 2537 A. The ultraviolet radiation is converted by the phosphor coated internally on the walls of the envelope into visible light which is transmitted through the glass walls. f

The electron temperature is the most important parameter in determining the ultraviolet radiation produced in the low pressure mercury vapor discharge. It determines the fraction of electrons having suflicient energy to excite mercury atoms from the ground state into excited states from which they can radiate energy. There is an optimum mercury vapor pressure, usually about six microns, at which the efliciency of conversion of electrical energy into ultraviolet energy isa maximum with a falling off on-either side. However even when the mercury vapor pressure is maintained at the optimum, for instance by maintaining the entire envelope or if desired a selected restricted portion thereof at a corresponding temperature of approximately 40 C., the eificiency still decreases with increasing loading or power input.

The foregoing behavior of the low-pressure mercury vapor discharge can be explained on the basis of the increase in elastic collision losses in connection with the imprisonment of resonance radiation. A photon or quantum of radiation which originates in the body of the discharge cannot reach the walls directly but is repeatedly absorbed by mercury atoms and re-emitted in its random path to the wall. At each absorption, there is a finite probability that the excited mercury atom so produced will sufier an ionizing collision and thus lower the potential gradient of the discharge and fail to re-r'adiate. At the same time a substantial proportion of energy is being drained off into elastic collision losses resulting from the fact that when an electron strikes an atom and bounces. ofl elastically, that is without excitation or ionization of the atom, a small average fraction 1 of its energy is imparted to the atom,. given by where m is the mass of the electron and M that of the atom. While this fraction is exceedingly small, the rate of collisions with .gas atoms in fluorescent lamps is so large that the loss is substantial and results in heating of-the gas. Losses of the foregoing types increase with the number of starting gas atoms presentwhich depends upon the starting gas filling pressure, with the number of mercury atoms present which is governed by the mercury vapor density, and withthe number of electrons present which is governed bythe power input.

In order toincrease the rate of production of ultraviolet radiation, especially'at high loadings, that is at loadings of to 'watts per lineal foot of lamp and up, the electron temperature must be raised. This may be achieved by increasing the difiusion losses of ions to the wall inasmuch asincreased ion losses entailv higher electron temperatures in order to'produce replacement ions faster. An electric lampin which this isefiected and increasing ionic mobilities.

3,012,165. Patented Dec. 5, 196 1 advantageously is described in copending application Serial No. 577,017 filed April 9, 1956 by Eugene Lemmers, entitled Tubular Electric Lamps, and assigned to the same assignee as the present invention. In that lamp, the diffusion losses are increased through the use of a re-entrant groove cross section. Such a lamp has now been marketed for several years by the assignee of this invention and is well-known under the commercial designation Power Groove.

Another well-known device for increasing the wall losses whereby to achieve a higher electron temperat'ure consists in increasing the ionic mobility of the inert filling gas. The gases xenon, krypton, argon, neon and helium in the order named have decreasing atomic weights Thus by replacing the more commonly used argon by neon or, going still further, by helium, the ionic mobility is increased so that greater efliciency may be achievedif the loading is sufliciently high. Increasing the loading capacity by recourse" to a lighter fill gas such as neon cannot be as effective from the point of view of eificiency as recourse to a higher ratio of perimeter to cross section because, other conditions being equal, the efiiciency with a neon gas filling is less than with an argon gas filling at the same electron temperature. This follows from the fact that the ratio which determines the elastic collision loss'esis greater in the case of neon than for argon inasmuch as the denominator M (mass of the gas) is less in the case-of neon. A lighter fill gas such as neon has the disadvantage, by comparison With a heavier gas such as argon, that the positive ion attack of the cathodes is greatly increased resulting in early end darkening of the lamp and shorter life.

However if high loading or very high output of light is to be achieved in a fluorescent lamp of round cross section, that is without recourse to a higher ratio of circumference to cross section, then neon does'ofier some advantages albeit coupled with the disadvantages which have been pointed out. This explains the presence on the market in the past few years of a fluorescent lamp of 200 watts nominal rating in an 8' length of 1- /2" diameter (96T12) and which is claimed to operate at a loading of 25 watts per foot with a filling of neon. 'The life or maintenance of that lamp is known .to be very low. I At the lower and more conventional loadings of fluorescent lamps, for instance at loadings in the range of S'to' 15 watts per foot and centering about 10 watts per foot, it has for a long time been thought that argon is preferable as the fill gas or possibly one of the yet heavier inert gases such as krypton and xenon or a mixture of the gases argon, krypton and xenon. However in accordance with my invention, I have now discovered that quite unexpectedly at relatively low loadings in the range of 5 to 15 watts per foot an increase in efliciency is achieved by replacing up to approximately 50% of the usual argon filling gas by neon for a total pressure ranging from 1.5 to 3 millimeters. .The improvement in efficiency which may be effected thereby using the optimum mixture at any one total pressure is approximately 2% over the maximum efficiency which may be achieved with pureargon at the same total pressure. The proportion of neon which may beneficially be added is less at the smaller total pressure than at the higher total pressure. For instance at a total pressure of 1.5 millimeters, 10% neon is preferred whereas at a total pressure of 2 millimeters of mercury, 30% neon is preferred,;and at a total pressure of 3 millimeters, as much as 50% may be preferred. I

For, more details of the invention, attention is now directed to the following description and accompanying drawing. The features of the invention believed to be novel will be more particularly pointed out in the ap pended claims.

In the drawings:

FIG. 1 is a side elevational view of a gaseous discharge lamp embodying the invention.

FIG. 2 is a plot of luminous efiiciency vs. bulb wall temperature for a gas filling of argon with various percentages of neon at a total pressure of 1.5 millimeters.

FIG. 3 is a similar plot to that of FIG. 2 wherein the total gas pressure is 2 millimeters.

Referring to FIG. 1, the low pressure gaseous discharge lamp 1 selected as an embodiment of my invention may be, as regards its general configuration and structure, like an ordinary 40-watt rapid start fluorescent lamp of 48" nominal length and 1 /2" nominal diameter. The lamp comprises an elongated cylindrical envelope 2 having shouldered ends to which are secured bases 3 each provided with a pair of insulated contact terminals or pins 4. As shown at the ends of the lamp where a fragment'of the envelope wall has been broken out, the electrode mount or stem flare 5 is sealed peripherally into the shouldered tube end and includes a press 6 through which are sealed current inlead wires 7. The inward projections of the lead wires support the filamentary cathode 8, whereas the outward projections are connected to the terminal pins 4. Cathode 8 may consist of a coiled-coil of tungsten wire provided with an overwind and coated with an activated mixture of alkaline earth oxides such as the usual mixture comprising barium and strontium oxide. The other end of the lamp is provided with a similar cathode and one of the stem flares is provided with an exhaust tube which is sealed or tipped ofi in the usual fashion. Preferably and as illustrated, the cathodes are surrounded by shields or anode plates each comprising a pair of L-shaped sheet metal plates 9 welded to the ends'of the inlead wires .7. These shields are of substantial area in order to reduce. the anode voltage drop and are at a distance from the emitting surface of the cathode greater than'approximately 3 millimeters in accordance with the teachings of copending application Serial No. 812,235 of Eugene Lemmers filed concurrently herewith, entitled Low Pressure Discharge Lamp and assigned to the same assiglee as the present invention. One of the shield plates has been removed at the right hand end of the lamp in the drawing for ease of illustration.

- 'The lamp contains. a quantity of mercury indicated by the droplet 11 exceeding in amount the quantity vaporiz'ed during operation. The lamp contains in addition a ing at loadings of about watts per foot has been to It has been use argon at a pressure of 3 millimeters. known that some improvement in efliciency could be achieved by lowering the argon filling gas pressure to the range of ?/4 to 1 millimeter. However such a practice has not .foun'd'commercial acceptance because it entails a reduction in life of the lamps by reason of the increased attack upon the cathodes attendant upon the use or such ;a low filling gas pressure. The inert filling gas may be viewed as a protective blanket around the cathodes and its protective effect increases in accordance with the atomic weight of the gas selected and its pressure.

ance with the invention, I have found that a decided improvement in efiiciency without undue curtailment of life may be achieved by'replacing up to approximately In accord 50% of the argon filling gas by neon within the pressure range of 1.5 to 3 millimeters. Thus it is not necessary to go as low as 1 millimeter when pure argon is used and a substantial increase in efiiciency is achieved while yet maintaining an adequate cathode life.

Referring to FIG. 2, curves 14, 15 and 16 illustrate the variation in luminous efliciency against envelope wall temperature for a filling of argon with 10%, 30% and 50% neon respectively, at a total pressure of 1.5 millimeters. To afford a basis of comparison, curve 17 shows the variation in luminous efficiency with wall temperature for the conventional gas filling of argon at 3 millimeters pressure. The luminous efiiciency with an argon gas filling of 3 millimeters pressure and a wall temperature of 40 C. (corresponding to a mercury vapor pressure of 6 microns) is taken as The plotted data were taken from tests made with a single lamp attached to an all glass system with diiferent gases available. The control of the mercury vapor pressure over the range from 5 to 50 C. was accomplished with the aid of a refrigeration unit cooling a water bath over the desired temperature range, the bath being kept well stirred to maintain a uniform water temperature around the lamp envelope. The data were measured after the lamp had been burned for about hours in order to get away from the first period of fast depreciation. It will be observed that in an argonneon mixture at a total pressure of 1.5 mm., maximum efiiciency is achieved with the lowest proportion of neon, namely 10% neon.

Referring to FIG. 3, curves 18 to 20 show the variation in luminous efficiency vs. envelope wall'temperature for fillings of argon with 10%, 30% and 50% neon respectively at a total pressure of 2 millimeters. Again for the purpose of affording a basis of comparison, curve 17 shows the variation of luminous efficiency with wall temperature for a conventional filling of argon at 3 millimeters. It will be observed that the maximum efficiency in this case wherein the total filling gas pressure is 2 millimeters occurs with a 30% neon admixture.

A summary of the results of these tests preceded by similar tests conducted with pure argon at pressures of 1.5, 2 and 3 millimeters of mercury and giving the watts input, voltage, relative light output, and relative luminous efficiency in percent lumens per watt is given in Table I which follows:

Table I Pressure, Gas Watts Volts Light, L /W,

mm. Hg percent percent 1. 5 30% Ne 39 103 107 105. l

l. 5 50%Ne- 40. 8 108. 5 107. 0 101. 5

:ment in efiiciency in low pressure mercury vapor discharge lamps operating at conventional loadings in the range of 5 to 15 watts per foot length 'andgenerally about 10 watts per foot; For a total filling pressure 'of 3 millimeters of. mercury, an improvement in efiiciency has been observed with admixtures of up to 50% neon.

Maximum-efiiciency'is achieved with a lesser percentage of neon as'the total pressure is reduced. Thus at a total pressure of 3 millimeters of mercury, maximum efiiciency is achieved with approximately 50% neon; at 2 millimeters, with approximately 30% neon; whereas at a total pressureof -1.5 millimeters of mercury, maximum efiiciency is achievedwith approximately 10% neon.- The preferred proportion of neon in accordance with the invention for lamps operating at loadings in the range of to watts per foot length therefore varies generally from 10% at the lower pressure limit of 1.5 millimeters of mercury, to 50% at the upper pressure limit of 3 millimeters of mercury. The choice of pressure in any particular case will depend upon the emphasis placed on efliciency which is favored by lower pressures, and the emphasis placed on life or maintenance which is favored by higher pressures. The preferred filling gas pressure in accordance with the invention for a 40-watt fluorescent lamp using a 4 foot long tube having a nominal diameter of 1 /2" and operating at a loading of approximately 10 watts per foot length consists of argon with to 40% neon, for instance neon, at a total pressure of approximately 2 millimeters of mercury. This choice represents a studied compromise between efliciency and long cathode life, the latter being of course essential to a commercially acceptable product. It will be observed that the use of argon with 30% neon at a total pressure of 2 millimeters of mercury achieves a gain of about 6% in efiiciency when compared with the heretofore conventional filling of pure argon at 3 millimeters pressure.

While the increase in efiiciency upon the admixture of neon to argon at low pressure and relatively low loadings was quite unexpected, there appears to be a rational explanation for it. It has been established that the maximum efficiency for pure argon is in the range of to 1 millimeter pressure. Now neon in and of itself is intrinsically less efficient than argon at lower loadings due to the higher loss per elastic collision given by the factor where m is the mass of the electron and M is the mass of the neon atom. However when neon is added to argon and a total pressure is maintained constant, the partial pressure of argon is reduced and drops towards the optimum for argon alone occurring in the range of A to 1 millimeter. This would also explain why maximum etficiency is achieved with a lesser proportion of neon at the lower total pressure of 1.5 millimeters. The advantages realized by the invention are not dependent of course upon the validity of this suggested explanation.

A further benefit of the admixture of neon in a lamp in accordance with the invention is the fact that the admixture of neon raises the voltage drop across the lamp and therefore increases the power input into the lamp which is provided by a given current. Thus referring to Table I, it will be seen that reducing the starting gas pressure where pure argon is used reduces the voltage drop and the watts input into the lamp. However by using an admixture of neon, the voltage drop and the power input are raised back towards the desired level. Thus the admixture of 30% neon with argon at a total pressure of 2 millimeters results in a voltage drop of 104.8 volts and a power input of 39.5 watts corresponding substantially to the nominal 40-watt rating of the lamp.

While a certain specific embodiment of the invention has been illustrated and described in detail, various modifications will readily occur to those skilled in the art. The appended claims are therefore intended to cover any such modifications coming within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A low pressure electric discharge lamp comprising an elongated envelope having a pair of electrodes sealed into opposite ends, and containing an ionizable medium operable at a loading in the range of 5 to 15 watts per foot length of said envelope and comprising a small quantity of mercury in excess of that vaporized during operation at said loading and a filling of an inert starting gas mixture at a totai pressure between a lower limit of 1.5 millimeters of mercury and an upper limit of 3 millimeters of mercury and consist ng of argon with a proportion of neon varying generally from 10% at the lower limit to 50% at the upper limit.

2. A low pressure electric discharge lamp comprising an elongated envelope having a pair of electrodes sealed into opposite ends, and containing an ionizable medium operable at a loading of approximately 10 watts per foot length of said envelope and comprising a small quantity of mercury in excess of that vaporized during operation at said loading and a filling of an inert starting gas mixture at a total pressure between a lower limit of 1.5 millimeters of mercury and an upper limit of 3 millimeters of mercury and consisting of argon with a proportion of neon varying generally from 10% at the lower limit to 50% at the upper limit.

3. A low pressure electric discharge lamp comprising an elongated envelope having a pair of electrodes sealed into opposite ends, and containing an ionizable medium operable at a loading of approximately 10 watts per foot length of said envelope and comprising a small quantity of mercury in excess of that vaporized during operation at said loading and a filling of an inert starting gas mixture at a total pressure of approximately 2 millimeters of mercury and consisting of argon with 25 to 40% neon.

4. A low pressure electric discharge lamp comprising an elongated envelope having a pair of electrodes sealed into opposite ends, and containing an ionizable medium operable at a loading of approximately 10 watts per foot length of said envelope and comprising a small quantity of mercury in excess of that vaporized during operation at said loading and a filling of an inert starting gas mixture at a total pressure of approximately 2 millimeters of mercury and consisting of argon with approximately 30% neon.

5. A low pressure electric discharge lamp comprising an elongated envelope having a pair of electrodes sealed into opposite ends, and containing an ionizable medium operable at a loading of approximately 10 watts per foot length of said envelope and comprising a small quantity of mercury in excess of that vaporized during operation at said loading and a filling of an inert starting gas mixture at a total pressure of approximately 3 millimeters of mercury and consisting of argon with approximately 50% neon.

6. A low pressure electric discharge lamp comprising an elongated envelope having a pair of electrodes sealed into opposite ends and containing an ionizable medium operable at a loading of approximately 10 Watts per foot length of said envelope and comprising a small quantity of mercury in excess or" that vaporized during operation at said loading and an inert starting gas mixture at a total pressure of approximtely'lfi millimeters of mercury and consisting of argon with approximately 10% neon.

References Cited in the file of this patent UNITED STATES PATENTSv 1,951,006 Balcar Mar. 29, 1934 2,346,522 Gessel Apr. 11, 1944 2,444,397 Cram June 29, 1948 2,622,221 Beese Dec. 16, 1952 2,687,486 Hiene et al Aug. 24, 1954 2,714,682 Meister et a1 Aug. 2, 1955 2,802,129 Meister et al. Aug. 6, 1957 FOREIGN PATENTS 748,890 Great Britain May 16, 1956 

